RSV is the major cause of serious lung infections in infants. During the first 12 months of life, it's the most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in the U.S.

Low vitamin D levels have been suspected of playing a role in vulnerability to RSV. This led Mirjam E. Belderbos, MD, of the Utrecht University Medical Center in the Netherlands, and colleagues to measure vitamin D levels in the cord blood of 156 newborns and then to follow the children for one year.

At birth, more than a quarter of the infants had low vitamin D -- serum levels of less than 20 ng/mL. During their first year of life, these kids had a sixfold higher risk of RSV lung infection than did the 46% of kids whose vitamin D levels at birth were at least 30 ng/mL.

"We demonstrated that 54% of healthy newborns in the Netherlands are born with insufficient [vitamin D] concentrations required for maximum health, and that low plasma concentrations of [vitamin D] are associated with increased risk of RSV lower-respiratory tract infections in the first year of life," Belderbos and colleagues report.

It's not just the Netherlands. Other Western nations, including the U.S., have similar rates of low vitamin D.

U.S. researchers reported in 2010 that at a single Boston hospital, 58% of infants and 36% of mothers had low vitamin D levels (under 20 ng/mL). Severe vitamin D deficiency (defined as lower than 15 ng/mL) was seen in 38% of the infants and in 23% of the mothers.

Low Vitamin D and Childhood Disease

Vitamin D is a hormone that the body makes when exposed to direct sunlight. Belderbos and colleagues found that infants born in July had the highest vitamin D levels, while those born in December had the lowest levels.

RSV isn't the only problem for kids with low vitamin D. Vitamin D deficiency -- levels below 12 ng/mL -- causes the soft, weak bones of rickets. Rickets was common in the days before vitamin D was added to milk.

How can vitamin D affect infections? Belderbos and colleagues suggest three ways:

Vitamin D plays a role in the development of the fetal and infant immune system.

Vitamin D may accelerate early lung development.

Vitamin D has antimicrobial properties.

Just under half of the women in the Dutch study took vitamin D3supplements. Infants born to these women had significantly higher vitamin D levels at birth. However, the study was too small to show whether maternal vitamin D supplements protected infants from RSV lung infection.